Carbon Nanotube Composite Membrane

ABSTRACT

A composite membrane for separations includes a fabric with a non-woven array of intermingled carbon nanotubes, and a dopant incorporated with the fabric to form a non-porous, permeable composite. The composite membrane may be used to separate a target gas from a liquid by mounting the composite membrane in a housing chamber, and conditioning a permeate side of the chamber to establish a driving force for the target gas across the non-porous, permeable composite membrane.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 13/911,433, filed on Jun. 6, 2013 and entitled “Carbon Nanotube Composite Membrane”, the contents of which being incorporated herein in their entirety.

FIELD OF THE INVENTION

The present invention relates to gas separation and ion transport membranes generally, and more particularly to a composite membrane structure having a backbone of an array of nanotubes.

BACKGROUND OF THE INVENTION

A long list of polymers have been studied in the past for their utility in the formation of membranes for a variety of purposes. Of importance to the present invention is the use of such polymer membranes for separations and selective transport to modify a feed material and/or to recover one or more target species from the feed material. A host of polymers have been determined to be useful for such applications, with each material exhibiting its own benefits and drawbacks for particular membrane separation and transport applications.

An application of particular interest to the Applicant is the degassing of liquids through contact with a gas-permeable, liquid-impermeable membrane. Such liquid-gas contactors typically rely upon Henry's Law of partial pressures and Ficke's law of diffusion to drive gas transport through the membrane, while small pore size, or the absence of through-pores in a “nonporous” media, restricts or prevents liquid transport through the membrane. The development of fluoropolymers has greatly aided the membrane liquid degassing field by providing membrane polymers that are generally inert, and can be formed into a gas-permeable, liquid-impermeable membrane structure. A particular fluoropolymer of note is a class of amorphous perfluoropolymers, such as those available from Du Pont under the trade name “Teflon®”, as well as other amorphous fluoropolymers available from Asahi Glass Corporation and Solvay Solexis. Such materials are oftentimes employed in gas separation membranes for their inertness and high permeability characteristics. Membranes are typically selected for a combination of their compatibility with the contacting materials, their permeability to the targeted transport species, and their selectivity of one molecule over another. It has been shown that, while membrane selectivity may be constant as a function of the membrane thickness, the throughput (permeance) changes inversely to the thickness of the membrane. As a result, a thinner membrane is typically desired, but is limited by the decreased strength and durability as membrane thickness is reduced. It is therefore an ongoing challenge to obtain selective membranes that have the highest possible permeance without being unduly fragile. Such membranes should also be resistant to fouling, degradation, or other performance deterioration.

Membrane engineers have attempted to employ fragile membranes with desired performance properties by supporting the selective membranes with a support structure. A variety of reinforcing support structures have been previously implemented, but are typically difficult to handle, expensive, and/or degrade the performance of the primary selective membrane. Suitable structural reinforcements to thin film membranes that avoid these drawbacks have yet to be defined.

Reinforcement materials for thin film membranes have typically been in the form of lattice structures, support films, and particulate dopants. One material that has been extensively studied for its strengthening properties, though not in thin film separation membranes, is carbon nanotubes, which are recognized as a high-strength material, deriving their strength from its native sp² bond structure. The electron cloud associated with the sp² bonding structure functions as an interaction between proximate carbon nanotubes, such that nanotubes may be formed into coherent sheets, tapes, ribbons, ropes, and other macrofabrics, with a tensile strength that is sufficient to facilitate handling.

Internanotube forces have been noted so long as the nanotubes are well associated wherein the surfaces of proximate nanotubes can interact. The particular method of forming nanotubes into such sheets, tapes, and the like, however, can greatly affect the strength of the so-formed macro-scale nanotube structures. Carbon nanotube arrays in a sheet form are commonly known as “buckypaper”, which owes its name to buckminsterfullerene, the 60 carbon fullerene (an allotrope of carbon with similar bonding that is sometimes referred to as “Bucky ball” in honor of Buckminster Fuller). Generally, the bonding interactions among the nanotubes are insufficient to form a buckypaper that has commercial use on its own. However, carbon nanotubes have been described as a dopant to various materials, including polymers, by mixing carbon nanotube powder into the polymer. Typically, researchers seek improved strength and/or electrical conductivity when doping polymers with carbon nanotubes.

Strength reinforcement materials, including glass fibers, carbon fibers, metal fibers, carbon nanotubes, and the like, when conventionally added as a reinforcement material, are dependent upon surface energy compatibility between the reinforcement material and the matrix for the degree of strength enhancement. Matching of the respective surface energies permits van der Walls interactions to assist in the load transfer between the reinforcement material and the matrix. In some cases, surface energy matching is not possible without chemical modification of the reinforcing material, which chemical modification can be expensive or even impossible.

Recently, a reverse approach has been attempted, wherein a buckypaper is infused with a polymer to maintain the native strength of the carbon nanotube sheet derived from the van der Walls interactions among the nanotubes. An example of such an approach is described in U.S. Pat. No. 7,993,620, herein incorporated by reference. The non-woven carbon nanotube fabric described in U.S. Pat. No. 7,993,620 may be incorporated into composite structures by impregnating the non-woven fabric with a matrix precursor, and allowing the matrix to polymerize or thermally cure. Such composites have been described for use in impact-resistant applications, such as sporting goods protection devices, including helmets. Other carbon nanotube fabric composites are described in U.S. Patent Application Publication No. 2010/0324565, also incorporated herein by reference.

The example carbon nanotube fabric structure described above for the formation of composites is described in detail in U.S. Patent Application Publication Nos. 2009/0215344, and 2011/0316183, while an apparatus useful to synthesize nanotubes of such carbon nanotube fabrics is described in U.S. Patent Application Publication No. 2009/0117025, each of which are incorporated herein by reference.

Though composites of carbon nanotube fabrics and infused polymers have been demonstrated, the Applicant is unaware of such composites prepared as a thin film membrane for, as an example, separations. One explanation for the lack of work in this area may be due to the expectation that carbon nanotube fabrics would act similarly to other reinforcement structures that interfere with the overall permeability of the composite structure. It is well know that solid portions of conventional thin film support structures often reduce permeance performance as compared to the neat thin film separation membrane.

It is therefore an object of the present invention to provide a composite structure that exhibits desired tensile strength with a substantially reduced effective polymer film thickness.

It is another object of the present invention to provide a composite membrane incorporating a nanotube reinforcement structure that does not significantly degrade permeation performance of the separation polymer matrix.

SUMMARY OF THE INVENTION

By means of the present invention, a thin membrane for separations may be prepared with significantly less polymer material while maintaining, or even enhancing strength properties of the membrane. By reducing the polymer usage, separation membrane cost may correspondingly be significantly reduced. The reinforcement structure of this composite membrane is a non-woven carbon nanotube fabric that is single or multiple layers, while the carbon nanotubes are intermingled.

In one embodiment, a method for preparing a composite gas separation membrane for separating a gas-liquid mixture includes providing a fabric having a non-woven array of intermingled carbon nanotubes, wherein the non-woven array defines interstices between the intermingled carbon nanotubes. The method further includes providing a dopant and at least partially immersing the fabric in the dopant. The dopant is sonicated with an ultrasonic transducer such that the dopant penetrates the fabric interstices to an extent sufficient to establish a non-porous but permeable composite structure with the fabric.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a testing apparatus for testing permeability, permeance, and gas selectivity of composite films of the present invention;

FIG. 2 is a cross-sectional scanning electron microscopy (SEM) image of a composite film of the present invention;

FIG. 3 is a cross-sectional SEM image of a composite film of the present invention;

FIG. 4 is a schematic illustration of a membrane separation system of the present invention;

FIG. 5 is a schematic illustration of a membrane ion exchange system of the present invention;

FIG. 6 is a photograph of a cross-section of a sample composite membrane taken under scanning electron microscopy;

FIG. 7 is a spectral analysis chart illustrating the presence of carbon, fluorine, and oxygen at respective spectral analysis locations identified in the photograph of FIG. 6;

FIG. 8 is a spectral analysis chart illustrating the presence of carbon, fluorine, and oxygen at respective spectral analysis locations identified in the photograph of FIG. 6; and

FIG. 9 is a tensile strength chart illustrating Young's Modulus values for various sample composite membranes, in comparison to a nonwoven carbon nanotube fabric labeled “Pure”.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects, features, and advances represented by the present invention will now be presented in terms of detailed embodiments. Other embodiments and aspects of the invention are recognized as being within the grasp of those having ordinary skill in the art.

For the purposes hereof, the term “nanotubes” is intended to mean single wall or multiple wall tubular structures having a diameter in the range of about 1-100 nanometers (nm), and a length in the range of about 0.001-10 millimeters (mm). The nanotubes described in the experimental section hereof are multiple wall carbon nanotubes typically having a diameter of between about 1-25 nanometers (nm), and a length of between about 0.1-5 millimeters (mm). Both single wall and multiple wall carbon nanotubes, however, are well understood in the art and are contemplated as being useful in the present invention.

For the purposes hereof, the term “membrane” is intended to mean a barrier structure that is capable of permitting selective transport thereacross. A “composite membrane” is a membrane structure having two or more bodies coordinating with one another in a single membrane structure. The bodies may be comprised of the same or different materials, and are exemplified in this description as a polymer matrix body incorporated with a carbon nanotube fabric.

For the purposes hereof, the term “non-porous” is intended to mean that the subject composite film is substantially free from open pathways extending continuously from one side of the composite film to the other. A “pore” is defined as an opening by which matter passes through a wall or membrane. Thus, the term “non-porous” is intended to mean the absence or substantial absence of pores. With regard to carbon nanotubes, a “pore” could be defined as a lumen of a carbon nanotube that extends through the thickness of the composite membrane to provide a pathway through the composite membrane. The non-porous composite membranes of the present invention are free from such lumenal pathways through the composite membrane.

For the purposes hereof, the term “permeable” is intended to mean transport through the composite membrane that relies primarily upon a solution-diffusion mechanism, and not as a result of porosity in the structure permitting material transport through the composite membrane without a solution-diffusion mechanism.

As described above, carbon nanoparticles structurally similar to nanotubes have been employed in numerous applications, primarily as a mechanical strengthening agent. In some of such applications, however, the addition of carbon nanoparticles in the form of graphene has resulted in a significant reduction in polymer film permeability. Indeed, nanoparticles have been demonstrated and intentionally used to limit permeability in a substrate, such as in U.S. Pat. No. 7,745,528 and U.S. Patent Application Publication No. 2007/0137477. Consequently, the Applicant's finding that a carbon nanotube support structure to a polymer film does not significantly inhibit permeability performance is a surprising result of the present invention.

A “buckypaper” found by the Applicant to be useful in the composite membranes of the present invention are available from Nanocomp Technologies, Inc. of Merrimack, N.H. The Nanocomp buckypapers are described in, for example, U.S. Patent Application Publication Nos. 2011/0316183 and 2009/0215344, as well as U.S. Pat. No. 7,993,620 as a non-woven fabric of intermingled nanotubes generated through chemical vapor deposition (CVD) or other gas phase pyrolysis procedure, the contents of which patents and patent applications being incorporated herein by reference. The intermingled nanotubes are randomly dispersed in a random orientation in the non-woven fabric, with the fabric being comprised of multiple layers of randomly-oriented non-woven nanotubes that, taken together, form the carbon nanotube fabric. The nanotubes making up the tested non-woven fabric included multiple wall nanotubes having an outside diameter of approximately 10-15 nm, and a length of between about 0.1-5 mm randomly arranged within layers of a multiple layer non-woven fabric weighing between 1-20 g/m². The example nanotube fabrics have between 5-95% open structure, and more preferably between 30-70% open structure, with a thickness of between 1-50 micrometers.

For the purposes hereof, the term “nonwoven fabric” is intended to mean a sheet, web or bat of directionally or randomly oriented fibers, bonded through various means, including friction and/or cohesion and/or adhesion. It is believed that the carbon nanotube fabrics described above are bonded through π-π electron cloud interaction among the carbon nanotubes. It is contemplated that the fabric may include partially or completely directionally oriented fibers, or may instead be randomly oriented nanotubes.

A wide host of dopants may be incorporated with the nanotube fabric to form the composite membranes of the present invention. For the purposes hereof, the term “dopant” is intended to mean a material that is capable of penetrating into interstices of the non-woven fabric, and which is capable of forming a network that is sufficient to establish a non-porous but permeable barrier. Example dopants include monomers/polymers in neat form or in solution. The polymers described in the following examples were selected for their inertness and relatively high gas permeabilities. It is to be understood, however, that other materials, including other polymers, polymer blends, and non-polymers may be utilized in the formation of the composite membranes of the present invention. The selected dopant material is preferably capable of depositing within the open interstitial space of the nanotube fabric to an extent to form a nonporous but permeable composite structure having a target gas permeance of at least 0.1 gas permeation units (GPU). In this manner, the composite membrane may be useful in a variety of separations applications, including gas-gas, gas-liquid, and liquid-liquid separations.

The term “incorporated with” is intended to describe the relationship between the dopant and the nonwoven fabric, wherein the dopant is not merely deposited upon or adjacent to the nonwoven fabric surface, but additionally or instead penetrated into interstices of the nonwoven fabric to the extent that, upon any necessary polymerization or cure of the dopant, the dopant and the nanotubes interact as a composite membrane exhibiting significantly greater absolute strength (Young's modulus) than the respective absolute strengths (Young's Modulus) of either of the polymerized/cured dopant or the nonwoven fabric alone. Such synergistic strengthening of the composite membranes of the present invention is a surprising and important aspect, and defines the extent of penetration of the dopant into interstices of the nonwoven fabric necessary in order to be “incorporated with” the nanotube fabric for the purposes of the present invention.

Penetration may be defined as the distance that the dopant penetrates into the nonwoven fabric, and can be inferred in the following examples by measuring the fluorine concentration through the thickness of the dried composite membrane in the case of a fluoropolymer dopant. It has been determined that, in order to form the composite membranes of the present invention, the dopant must penetrate into the surface of the nanotube fabric. Penetration of the dopant into the nonwoven fabric depends upon the viscosity of the dopant, including as solvated in solution, and the surface energy difference between the dopant/dopant solution and the nonwoven fabric. Penetration is also dependent upon the molecular size of the dopant.

Dopant penetration as measured by energy dispersive x-ray analysis through the thickness of the membrane monitoring fluorine concentration showed a strength to depth of penetration relationship consistent with the relative depth of dopant penetration. Dopant impregnations resulting in a substantially uniform dopant dispersion throughout the fabric exhibit the highest absolute strengths. However, it has been found that sufficiently strong, permeable, but nonporous composite membranes may be formed when only one side of the nonwoven fabric is exposed to the dopant/dopant solution. In the case of a dip tank, one side of the fabric may be protected from exposure to the dopant solution by use of a solid, impermeable barrier to which the dopant solution will not permanently adhere. The frame-attached nonwoven fabric is then exposed to the dopant solution for a period of time necessary to provide sufficient penetration into the nonwoven fabric. One benefit of such one-side impregnated fabrics, wherein the non-impregnated side of the fabric remains free of dopant, is that gasses permeating from the impregnated side of the membrane to the non-impregnated side may be vented to atmosphere, removed, by vacuum, or purged from the non-impregnated side of the membrane using gas pressure.

Typical composite membranes of the present invention permit at least 90% of mass transport only through a solution-diffusion mechanism, and more preferably at least 95%, and still more preferably at least 99% of the mass transport through the composite membrane only by a solution-diffusion mechanism. Such a composite membrane preferably transports sufficient mass through the membrane per unit area as is required for a given application.

The permeability, permeance, and gas selectivity measurements of target gasses, such as nitrogen, oxygen, and carbon dioxide, may be obtained using a constant volume/variable pressure apparatus. An example testing arrangement is illustrated in FIG. 1, wherein a testing apparatus 10 includes a cell 12 having a chamber 14 that is divided into an upper compartment 16 and a lower compartment 18 by the membrane 20 being tested. The tested membranes were each 0.95 cm² held in place by a clamp 22. Upper compartment 16 had a volume of greater than 1000cm³, while lower compartment 18 had a volume of 25 cm³. To test membrane 20, both compartments 16, 18 are first purged with an identical gas for about 30 min., followed by an additional 100 kPa of the gas added to upper compartment 16. The pressure of both compartments 16, 18 is measured with two Omegadyne model PX 209-015G5V pressure transducers as a function of time. The pressure of upper compartment 16 does not change during the test. When the rate of pressure increase on the lower compartment 18 reaches its pseudo-steady-state, the permeability was calculated as follows:

$P = {\frac{(22414)(l)(V)}{(A)\left( {\delta \; P_{0}} \right)({RT})} \times \frac{p}{t}}$

Where δP₀=initial pressure difference between upper compartment 16 and lower compartment 18 (cm Hg)

-   -   V=volume of lower compartment 18 (cm³)     -   l=thickness of membrane 20 (cm)     -   A=area of membrane 20 (cm²)     -   T=absolute temperature (K)     -   R=universal gas constant

$\frac{p}{t} =$

the rate of pressure increase on lower compartment 18 at pseudo-steady-state (cm Hg/sec.)

Tensile strength testing was performed using an Instron model #3345 equipped with a 1000 Newton or 5000 Newton load cell and a computer system operating Instron series IX/S software. Corrections for Poissons ratio were not made.

TABLE 1 Specimen Specimen Tensile Measurement Thickness, width length Pull Rate Strength Case Manufacturer Type Source (microns) (mm) (mm) (mm/min) (GPa) 1 Nanocomp Generic Nanocomp Generic Unknown Unknown 1.27 0.329 Thickness (mean value) 2 Nanocomp 20 Instron 72 25.4 25.4 1.27 0.92 gram/m² 3 Nanocomp 10 Instron 25 25.4 25.4 1.27 0.229 gram/m² 4 Nanocomp 5 Instron 9 25.4 25.4 1.27 0.307 gram/m²

Tensile strengths approaching those reported by the manufacturer were observed as the thickness of the nanotube fabric decreased. This may be due to the many layers of material used to make the higher weight fabrics and the apparent reduced density.

To determine the improvement in tensile strength achieved by incorporating polymers with the nanotube fabrics, the tensile strength of the individual polymers were tested and compared to literature values where available. Extruded polymer film was obtained from BioGeneral, Inc. of San Diego, Calif.

TABLE 2 Specimen Specimen Tensile Measurement Thickness width length Pull Rate Strength Manufacturer Type Source (microns) (mm) (mm) (mm/min) (GPa) DuPont Teflon Cast film Ca. 100 Hysitron TI 750 nanoindenter* 1.7 AF ™ 2400 DuPont Teflon BioGeneral 72 25.4 25.4 1.27 1.7 AF ™ Extruded 2400 Film Solvay Hyflon Cast film 25 25.4 25.4 1.27 0.53 Solexis AD-60 ™ DuPont Nafion ™ Literature − 9 7.35 × 10⁻⁴ 2020 film

Each of the polymers listed in Table 2 were prepared as impregnatable solutions as follows:

TABLE 3 Weight Type Polymer Solvent percent 1 Teflon AF ™ 2400 FC-770 ™ 3.5 2 Teflon AF ™ 2400 FC-770 ™ 2.5 3 Teflon AF ™ 2400 FC-770 ™ 1 4 Hyflon AD-60 ™ FC-770 ™ 2.5 5 Hyflon AD-60 ™ FC-770 ™ 10 6 Dupont Nafion ™ 2020 Neat 100 7 Dupont Nafion ™ 2020 Isopropanol 50

Various manual methods were used to impregnate the polymer solutions of Table 3 into the nanotube fabrics of Table 1. The following table sets forth initial observations of the composites.

TABLE 4 Polymer Fabric Solution type (from (from Composite Membrane Case Table 3) Table 1) Coating Method Drying Method Appearance 1 2 3 72 hour soak Ambient Stable, strong composite both wet and dry 2 2 4 72 hour soak followed by Ambient and Stable, strong composite mechanical impregnation oven at 60° C. both wet and dry 3 4 3 72 hour soak followed by Ambient and Stable, strong composite mechanical impregnation oven at 60° C. both wet and dry 4 5 4 72 hour soak followed by Ambient and Stable, strong composite mechanical impregnation oven at 60° C. both wet and dry 5 7 4 72 hour soak followed by Ambient and Stable, strong composite mechanical impregnation oven at 60° C. both wet and dry

The following examples illustrate composite membranes of the present invention, but are not intended to be limiting as to the materials and methods of formation for such membranes.

EXAMPLE 1

The composite membrane of Case 1 from Table 4 was prepared by dip coating the carbon nanotube fabric in the polymer solution for 72 hours while stretched over an aluminum frame. Following submersion for 72 hours, the composite material was withdrawn from solution and held in an atmosphere saturated with FC-770 solvent to prevent drying of the composite, and the excess solution was allowed to drain from the surface of the wet composite. The composite was then allowed to dry under ambient conditions under sufficient tension to remove wrinkles.

The dried composite membrane was then laser cut into one inch by two inch samples for tensile strength measurements, permeability and permeance testing, and scanning electron microscopy. The following Table 5 sets forth testing results of the composite membrane, in comparison to neat polymer films formed from Teflon® AF-2400 through extrusion, solution casting, melt pressing, and laboratory bench casting.

TABLE 5 Permeance (GPU) (1.1 cm dia) Thickness Tensile Permeability, Barrer O2/N2 Type (Microns) (GPa) O2 N2 CO2 O2 N2 CO2 ratio Solution n/a — 1600 780 3900 — — — 2.05 Cast Melt — — 990 490 2800 — — — 2.02 Pressed Extruded 38 1.7 585 273 1500 1.6 .75 4.17 2.14 Cast 19 n/a 882 446 2080 46.6 23.5 109 1.97 Composite 34 5.5 1870 1040 3550 55 31 104 1.79 Membrane (TABLE 4, Case 1) Reduced 17 — 934 521 1772 Mathematically equivalent to 1.79 Film Data composite membrane

The solution cast and melt pressed data are found in the literature, while the extruded film was sourced from BioGeneral, Inc.

Each of the laser-cut composite membranes was weighed to the nearest 0.1 mg, and the thickness determined to the nearest micrometer.

To determine the “reduced film data”, the weight of the carbon nanotube fabric was subtracted from the total composite weight to determine the amount of polymer remaining within the composite structure. The weight of polymer was converted to an equivalent film thickness by density correcting the weight to volume using a factor of 1.67 grams of polymer per cm³. An equivalent film thickness was then determined by dividing the volume by the length and width of the composite membrane.

The composite exhibits a tensile strength greater than that of the neat polymer, and dramatically greater than expected under the “Rule of Mixtures”. Applicant believes that an unexplained interaction between the polymer and the carbon nanotubes is responsible for the unexpected strengthening property demonstrated in the results table.

A cross-sectional image of the composite is illustrated in FIG. 2, which shows an asymmetrically formed membrane at section “A” with air inclusions to a depth of approximately four micrometers. Section “B” is approximately 15 micrometers thick, and is the bulk of the composite, while section “C” is an out of focus inner edge of the cross-section.

To further examine the distribution of the polymer through the bulk of the composite, an elemental analysis was performed to account for the concentration of fluorine, which is only available from the polymer and the solvent, through little solvent was expected to remain in the composite post-drying. The elemental analysis results are set forth in the following table, wherein “layer 1” is the epoxy potting compound used to prepare the sample for ion milling:

TABLE 6 Thickness Car- Fluor- Oxy- Observed Material Layer (microns) bon ine gen Composition 1 8 80 0 10 Epoxy Potting Compound 2 4 30 70 0 Teflon AF 2400 + Fabric 3 15 80 20 0 Fabric + Teflon AF 2400 or FC-770 4 4 0 0 0 Unfocused inner edge

The composite of this Example 1 exhibits a surprisingly higher tensile strength than either the base carbon nanotube fabric or the polymer in a neat film format. Moreover, the permeability of the composite membrane is also surprisingly unobstructed by the carbon nanotube fabric, contrary to what would be expected under Nielsen's Model.

EXAMPLE 2

The composite membrane described in Case 2 of Table 4 was prepared by soaking the carbon nanotube fabric in solution for 72 hours. Following soak, the composite was pressed while in the presence of polymer solution, and subsequently placed on a glass surface. Excess polymer solution was removed from the composite using a squeegee while compressing the composite surface to ensure the polymer solution beneath the composite against the glass was minimized and to remove air bubbles regained between the glass surface and the composite membrane. The composite membrane was then dried at ambient, followed by oven drying at 60° C. for four hours.

The following table sets forth strength, permeability, and permeance results for the composite membrane:

TABLE 7 Permeance, (GPU) (1.1 cm dia) Thickness Tensile Permeability (Barrer) O2/N2 Type (Microns) (GPa) O2 N2 CO2 O2 N2 CO2 ratio Extruded 38 1.7 585 273 1500 1.6 .75 4.17 2.14 Cast 19 n/a 882 446 2080 46.6 23.5 109 1.97 Composite 13 5.96 1740 900 4590 134 70 353 1.93 Membrane (TABLE 4, Case 2) Reduced Equivalent n/a 940 490 2470 1.93 Film Data to 9 microns

The permeability of the composite membrane closely matches the solution cast film permeability described in Table 5 with reference to a Teflon® AF 2400 film, as that employed in the present Example. In this instance, the weight of polymer remaining in the composite membrane is equivalent of a 9 micrometer neat polymer film thickness.

An ion-milled portion of the composite membrane was further examined under electron microscopy. The impregnation technique followed in this example closely matches a cast film in O₂/N₂ selectivity, and yet is stronger. An image of the electron microscopy of this composite membrane is shown in FIG. 3. Elemental analysis of the layers illustrated in FIG. 3 is set forth in the following table:

TABLE 8 Thickness Car- Fluor- Oxy- Observed Material Layer (microns) bon ine gen Composition 1 8 80 0 20 Epoxy Potting Compound 2 <1 30 70 0 Teflon AF 2400 + Fabric 3 2 50 50 0 Fabric + Teflon AF 2400 or FC-770 4 4 0 0 0 epoxy

The elemental analysis shows a distribution of the polymer throughout the thickness of the composite membrane, though the polymer concentration in this Example 2 appears to be higher throughout the thickness of the composite membrane in comparison to the composite membrane of Example 1. The elevated polymer concentration may be attributed to the mechanical force applied to the composite membrane in the fabrication technique described in this Example 2.

EXAMPLE 3

A composite membrane in accordance with Case 3 of Table 4 was prepared using the procedure of Example 2. The tensile strength of the composite membrane was compared to a membrane cast from Solution 5 of Table 3:

TABLE 9 Tensile Membrane Strength (GPa) Notes Polymer only 0.99 Pure polymer (Table 3, Type 5) Composite Membrane 6.95 Hyflon AD-60 composite with (Table 4, Case 3) 10 gram/m² Nanocomp fabric

EXAMPLE 4

A composite membrane in accordance with Case 4 of Table 4 was prepared using the procedure set forth in Example 2, and compared to a polymer cast film of the solution of Table 3, Type 5. Permeation testing of the composite membrane exhibited low selectivity, due to the extremely low permeability of the utilized polymer.

TABLE 10 Permeance (GPU) (1.1 cm dia) Permeability, Barrer O2/N2 Type O2 N2 CO2 O2 N2 CO2 ratio Solution Cast 51.4 16.5 125 — — —   3:1 (TABLE 3 Type 5) Composite 80 60 190 4.9 3.7 11.5 1.3:1 Film (TABLE 4, Case 4)

The data from the “solution cast” neat polymer film was taken from literature.

EXAMPLE 5

A composite membrane in accordance with Case 5 of Table 4 was prepared using the procedure set forth in Example 2. The composite membrane was tested for strength in both dry and water-saturated conditions, and compared to films of the neat polymer.

TABLE 11 Tensile Membrane Type Strength (GPa) Notes Nafion 2020 (Table 3, Type 6) 7.35*10⁻⁴ Literature Value Nafion 2020 Cast (Table 3, 0.280 Cast from 100% Type 6) (dry) solution Composite Film (Table 4, 7.8 Dry Film Case 5) (dry) Composite Film (Table 4, 2.56 Water Saturated Case 5) (wet)

The polymer utilized in this example includes a sulfonated tetrafluoroethylene that is commonly used in ion exchange membranes to permit ionic species to migrate thereacross. The example composite membrane may therefore permit transport of cations thereacross, substantially to the exclusion of anions and electrons. Moreover, although the polymer is commonly used in ion conductive membranes, films formed from such polymer are also typically highly selective to carbon dioxide (CO₂), thereby rendering it a good candidate material for carbon sequestration applications. However, the dry composite membrane is not expected to demonstrate desired gas permeability or selectivity, because the sulfonic acid regions of the sulfonated tetrafluoroethylene polymer must be hydrated so that carbon dioxide can be transported as the dissolved carbonate base, CO₃ ⁻². The sulfonic acid portion of the polymer in the composite membrane may then carry the ionic species across the barrier.

EXAMPLE 6

To further enhance ion conductivity of the composite membrane of Example 5, a carbon nanotube fabric of Table 1, Case 4 was chemically modified in a closed flask with 100 mL reagent grade H₂SO₄ and heated to the sulfuric acid boiling point while covered. The temperature was then reduced to about 275° C., and the carbon nanotube fabric was allowed to react in the sulfuric acid for four hours. The mixture was then allowed to cool to room temperature, and was thereafter maintained at room temperature for 72 hours. The modified carbon nanotube fabric was removed from the sulfuric acid and rinsed with deionized water until the rinse water showed a pH>5.

Visual examination of the modified carbon nanotube fabric indicated that it was easily wetted by water. The modified carbon nanotube fabric was oven dried at 60° C. for four hours. The modified carbon nanotube fabric was then impregnated with polymer solution 7 in accordance with the procedure of Example 2.

The composite membrane was tested for strength in both a wet and dry state:

TABLE 12 Membrane Type Tensile Strength (GPa) Nafion 2020 (Literature) 7.35 × 10⁻⁴ Water wet Composite Film 2.6 Dry Composite Film 2.56

The composite membrane was also tested for permeability with wet paper towels placed in the space above and below the wet composite membrane to maintain humid conditions during permeability testing.

TABLE 13 Permeance, (GPU) Membrane Permeability, (1.1 cm dia) Thickness (Barrer) CO2/N2 Membrane Type Microns N2 CO2 N₂ CO2 ratio Solution Cast 38 1 300 — — 300:1 Water wet 11 4.7 224 0.4 20  50:1 Composite Membrane Reduced 4 18 850 Film Data

EXAMPLE 7

A composite membrane was prepared from a 5 g/m² nonwoven carbon nanotube fabric (Case 4, Table 1) and Teflon AF™ 2400 dopant. The carbon nanotube sheet was laser cut into 6 in×6 in squares for mounting over a steel frame. The dopant solution was prepared as 3% by weight Teflon AF™ 2400 polymer in Novec™ 7500 solvent from 3M Company of St. Paul, Minn. 3 kg of dopant solution was placed in an immersion tank, which was then placed in a water sonication apparatus. In this arrangement, the immersion tank was placed in a water sonication tank so that ultrasonic energy emitted by an ultrasonic transducer 314 may be transmitted from the water medium in the water sonication tank through the immersion tank into the dopant solution. The ultrasonic transducer was a Fischer Sonic Cleaner emitting at 400 W and 30 kHz frequency.

The water sonication tank may be heated to warm the dopant solution to a desired impregnation temperature, typically in the range of 60-90° C. For this application, dopant solution was warmed to 85° C. to achieve desired viscosity levels of the dopant solution.

Once the dopant solution was warmed to the temperature set point, sonication from the ultrasonic transducer was initiated. The framed carbon nanotube fabric was then immersed in the dopant solution in the immersion tank for sixty minutes, with a typical immersion time ranging from 30-120 minutes. Sonication was then ceased and the framed membrane was removed from the remaining dopant solution in the immersion tank, and allowed to dry at ambient.

A cross-section of an example composite membrane generated through sonicated impregnation of the dopant solution into a non-woven carbon nanotube fabric is illustrated in FIG. 6, identifying multiple locations at which spectral analysis was performed to determine relative elemental composition among carbon, oxygen, and fluorine. The presence of fluorine and oxygen in the spectral analysis locations within the composite membrane cross-section indicate the presence of the fluoropolymer dopant. The spectral analysis results are illustrated in FIGS. 7 and 8, illustrating a strong presence of fluorine and oxygen throughout the cross-section of the composite membrane. This evidences the fact that substantially complete dopant penetration through the fabric cross-section was accomplished with the sonication technique. The chart shown at FIG. 7 indicates analysis by Internally normalized Dispersive Energy X-Ray to show dispersion of the carbon, fluorine, and oxygen through the thickness of the composite membrane. Normalizing the concentration to a value of numeral 1 for each concentration of oxygen, fluorine, and carbon individually removes the bias of their individual responsive relative to that of carbon. The chart of FIG. 8 indicates relative weight percent contribution of each of carbon, fluorine, and oxygen at each spectral analysis location.

Young's Modulus of the composite membranes, as tested by ASTM D882-12, were compared to the nonwoven carbon nanotube fabric alone. The data is illustrated in FIG. 9, evidencing Young's Modulus values for the composite membranes of at least about 4 GPa, which is substantially greater than Young's Modulus value of the bare Nanocomp 5 g/m² density nonwoven carbon nanotube fabric, identified as “pure” at 0.5 GPa, or the neat Teflon AF™ 2400 membrane having a thickness substantially equivalent to the reduced film thickness of the composite membrane samples, at 1.7 GPa. Accordingly, the composite membranes of the present invention far exceed the expected Young's Modulus values of the respective components under the “Rule of Mixtures”.

Applications

An example application for the composite membranes of the present invention is in gas/gas, gas/liquid, and/or liquid/liquid separations. In one embodiment, a system 110 for separating a target gas from a liquid includes a housing 112 defining a chamber 114, and a composite membrane 116 separating chamber 114 into a permeate side 118 and a retentate side 120. Housing 112 includes an inlet 122 and an outlet 124 opening to retentate side 120 of chamber 114. Housing 112 may further include a gas port 126 opening to permeate side 118 of chamber 114. System 110 may include a pump 128 for evacuating permeate side 118 of chamber 114 through gas port 126, with pump 128 being fluidly connected to gas port 126 through, for example, pipe 130.

A liquidous fluid containing the target gas may be delivered to retentate side 120 of chamber 114 through inlet 122 via pipe 132. It is to be understood that the term “pipe”, as used herein, is not intended to be limiting, and may include any conveyance member of suitable size, configuration, and material to permit the conveyance of fluid to an intended destination.

Composite membrane 116 may be suitably configured to be capable of separating the target gas from the liquid at retentate side 120, and may preferably form a non-porous, permeable barrier exhibiting a permeance of at least 0.1 gas permeance units (GPU) to the target gas. As is well known in the art, separation of the target gas from the liquid at retentate side 120 can be driven by the conditions at permeate side 118 of chamber 114. Accordingly, permeate side 118 of chamber 114 may be conditioned to exert a first partial pressure of the target gas that is less than a second partial pressure of the target gas in the liquidous fluid in retentate side 120 of chamber 114. Pursuant to Henry's Law of partial pressures and Ficke's law of diffusion, the target gas may be driven across the barrier defined by the nonporous composite film 116. Pump 128 may be operated to condition permeate side 118 of chamber 114, such as by evacuating permeate side 118 to an extent at which the first partial pressure of the target gas is less than the corresponding partial pressure of the target gas in a liquidous fluid at retentate side 120. In this manner, separated target gas permeated into permeate side 118 may be removed from chamber 114 through gas port 126.

Permeate side 118 of chamber 114 may also or instead be suitably conditioned by passing a sweep fluid through permeate side 118, wherein the target gas partial pressure within the sweep fluid is less than the corresponding target gas partial in the liquidous fluid at retentate side 120, so as to generate a driving force for permeation of the target gas through composite membrane 116 to permeate side 118. Such a sweep fluid may be introduced to permeate side 118 through a port in housing 112.

The above example system is not intended to be limiting as to the structures and methods of separations contemplated in the present invention. Gas/gas, gas/liquid, and liquid/liquid membrane separation systems are well known in the art, and are considered to be applicable to the systems and methods of the present invention, which uniquely employs a composite membrane.

In some embodiments, a composite membrane 216 includes an ionomer that is capable of transporting ionic species from a first fluid to a second fluid on opposed sides of the barrier defined by the composite membrane 216. A system 210 for transporting ionic species across a barrier between a first fluid and a second fluid includes a housing 212 defining a chamber 214, and a composite membrane 216 defining the barrier and separating chamber 214 into a first side 218 and a second side 220. Composite membrane 216 preferably includes a fabric comprising a non-woven array of intermingled carbon nanotubes and a dopant incorporated with the fabric to form a nonporous permeable composite. In some embodiments, the dopant is an ionomer that permits transport of cations across the barrier, substantially to the exclusion of anions and electrons. Such an ion exchange apparatus may be operated by applying a driving force to chamber 214, wherein the driving force is effective to motivate ionic species across the barrier defined by composite membrane 216. In some embodiments, such driving force may be in the form of electrical current generated in chamber 214 between first and second electrodes 230, 232 of opposite polarity. As illustrated in FIG. 5, first and second electrodes 230, 232 are positioned at opposed sides of composite membrane 216, with a first electrode 230 disposed in first side 218 of chamber 214, and second electrode 232 being positioned in second side 220 of chamber 214. First and second electrodes 230, 232 are coupled to an electrical energy source 240 for generating a voltage potential and current passing between first and second electrodes 230, 232. An applied current across composite membrane 216 to motivate ionic species transport is well understood in the art for membrane-based ion exchange systems.

Other driving forces are contemplated as being useful in motivating ionic species transport across composite membrane 216 between a first fluid delivered to first side 220 of chamber 214 through inlet 222, and a second fluid delivered to second side 218 of chamber 214 of inlet 224. In one instance, such a driving force may be generated based upon the properties of the first and second fluids on opposed sides of composite film 216. Most typically, however, membrane-based ion exchange systems employ an applied electrical current as the force driving ion species transport across the membrane.

The invention has been described herein in considerable detail in order to comply with the patent statutes, and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use embodiments of the invention as required. However, it is to be understood that various modifications to the invention may be accomplished without departing from the scope of the invention itself. 

What is claimed is:
 1. A method for preparing a composite gas separation membrane for separating a gas-liquid mixture, said method comprising: providing a fabric comprising a non-woven array of intermingled carbon nanotubes, wherein said non-woven array defines interstices between said intermingled carbon nanotubes; providing a dopant; at least partially immersing said fabric in said dopant; and sonicating said dopant with an ultrasonic transducer such that said dopant penetrates said fabric interstices to an extent sufficient to establish a nonporous but permeable composite structure with said fabric.
 2. A method as in claim 1 wherein said nonporous composite structure exhibits a target gas permeance of at least 0.1 GPU.
 3. A method as in claim 1 wherein said fabric has a density of between about 1-20 g/m².
 4. A method as in claim 1 wherein said dopant is solvated in a dopant solution at a concentration of 1-50 weight percent.
 5. A method as in claim 4, including heating said dopant solution to between 60-90° C.
 6. A method as in claim 1 wherein said ultrasonic transducer emits sonic energy at a frequency of between 10-50 KHz.
 7. A composite gas-separation membrane, comprising: a substrate body having first and second generally opposed surfaces defining a thickness therebetween, said body comprising a non-woven fabric of intermingled carbon nanotubes defining interstices between said intermingled carbon nanotubes; and a polymer dopant distributed within the interstices and throughout said thickness to form a network that is sufficient to establish a non-porous but permeable barrier between said first and second surfaces.
 8. A composite gas-separation membrane as in claim 7 wherein said polymer dopant is substantially uniformly distributed throughout said substrate body. 